59-13-06 DOUGLAS: Applies to the Following Aircraft: DC-6A Serial Numbers 43296, 43297, 43817-43819, 43839-43841, 44063, 44064, 44069-44073, 44076, 44257. DC-6B Serial Number 43257-43259, 43261-43276, 43291, 43292, 43298-43300, 43518-43537, 43539-43547, 43549-43555, 43557-43564, 43738-43741, 43743-43746, 43748-43750, 43820-43822, 43824-43826, 43828-43834, 43836, 43837, 43842, 43844-43847, 44056-44062, 44080-44083, 44087-44089, 44102-44113, 44165-44168 and 44251. \n\n\tCompliance required as indicated. \n\n\tSeveral instances of lower front spar cap cracks have been reported. The cracks involved are located at wing Station 30 at the intersection of lower front spar cap aft tang and wing-to-fuselage attach angle. To date no cracks have been found in the lower center spar cap; however, due to the similarity of the structure, it is logical to assume that cracks can occur in this cap as well as in the lower front spar cap. To detect cracking of the lower front and center spar cap tangs at intersection with lower fuselage attach angle, the following must be accomplished on affected aircraft having in excess of 12,000 flying hours. \n\n\t(a)\tInspect lower front spar caps at nearest maintenance inspection period to 200 flight-hours unless similar inspection has been conducted within last 1,250 flying hours. \n\n\t(b)\tInspect lower front and center spar caps at maintenance inspection period nearest to each succeeding 1,250 flying hours. \n\n\t\t(1)\tAt first 1,250-hour inspection period, holes in aft tank of front spar lower cap and fuselage attach angle should be enlarged and new attachments installed (KIT "A" of Douglas SB A-821 or equivalent). \n\n\t\t(2)\tAt next regularly scheduled overhaul period, holes located in forward tank of front spar lower cap should be enlarged and new attachments installed (KIT "A" of Douglas SB A-821 or equivalent). \n\n\t(c)\tIn event spar cap cracking is found at the 200-hour initial or 1,250-hour repetitive inspection periods, temporary rework per drawing No. 3645935 (KIT "B"), or equivalent, may be accomplished. \n\n\tWith temporary rework installed, inspection must be repeated at the 1,250-hour intervals for a maximum of 3,200 flight-hours at which time permanent rework per drawing No. 5761922 (KIT "C"), or equivalent, must be accomplished. \n\n\t(d)\tAll aircraft not already reworked per (c) must have permanent rework per drawing No. 5761922 (KIT "C"), or equivalent, accomplished within the next 6,400 flying hours. \n\n\t(e)\tAfter installation of KIT "C", operators may revert to normal repetitive inspection periods not to exceed 3,200 flying hours. \n\n\t(Douglas Service Bulletin DC-6 No. A-821 dated March 19, 1960, covers this same subject.) \n\n\tNOTE: This AD is not presently applicable to aircraft Serial Numbers 43548 and 43152 since they are not currently under U. S. registry. However, compliance with this AD will be required at the time application is made for recertification of such aircraft in the U.S.